The work aims to develop a high precision and efficiency polishing technique for single-crystal silicon carbide surface. Single crystal silicon carbide was polished by a magnetorheological polishing method using a large polishing tool excited by an electromagnetic field. The self-developed electromagnet excitation device and magnetorheological polishing device were used for single-factor experiments. Effects of process parameters such as current intensity, working gap and polishing time on polishing performance were studied. The roughness of polished surface and its rate of change were analyzed to investigate the polishing effect. Under the process parameters of working gap of 1.4 mm and current strength of 12 A, the roughness value of the polished surface decreased with the increase of polishing time. After 60 min, the roughness value of the polished surface reached Ra 0.9 nm, and the rate of change reached 98.3%. The roughness of the polished surface decreased with the increase of the current, and increased with the increase of the working gap. Under optimized parameters of the working gap of 1.0 mm, the energized current of 16 A and the processing time of 40 min, an ultra-smooth surface of single crystal silicon carbide was obtained with surface roughness of Ra 0.6 nm. The sub-nanometer surface roughness of single crystal silicon carbide materials can be obtained by the method of magnetorheological polishing using large polishing tool excited by electromagnetic field.